This invention relates generally to absorbed electrolyte batteries and, more particularly, to a sealed battery adapted to compensate for internal pressure variations.
A well known example of such batteries is the sealed lead-acid type although other types of absorbed electrolyte batteries are available, such as the nickel-cadmium configuration. Sealed lead-acid batteries, as an example of the type under consideration, typically have certain features in common. A common gas manifold system interconnects all cells and a venting device is normally provided to prevent excess gas pressure buildup within the battery. The battery elements are housed within a rectangular container which is divided by partition walls into a series of cells. An electrode stack is closely fitted within each cell. The electrode stacks comprise alternate positive and negative plates with separators interposed between the positive and negative plates.
In sealed lead-acid batteries, there is substantially no free unabsorbed electrolyte in the cells. The major portion of the electrolyte is restrained in the highly absorbent microfine glass fiber separator material between the positive and negative plates and within the pores of the positive and negative active material of the plates.
Although electrolyte is immobilized and absorbed in special separators, the separators are not fully saturated so that the gasses evolved during charging or at other times can diffuse rapidly from one electrode to the other. Thus, in what is termed an "oxygen cycle," oxygen is produced at the positive electrode and diffuses to the negative electrode where it rapidly reacts to combine with active lead. Effectively, this reaction partially discharges the negative electrode, preventing the negative electrode from reaching its fully charged state, thereby minimizing the evolution of hydrogen. When the oxygen reacting at the negative electrode is equal to or greater than the rate of oxygen being produced at the positive electrode, water loss through electrolysis and, more importantly, pressure build up are minimized.
However, the oxygen cycle takes place only under the following conditions. First, both the positive and the negative plates must be in intimate contact with the separator material so that the entire surface of the plates has adequate electrolyte for its electrochemical requirements. Thus, it is of paramount importance that the cells be maintained under a compressive force to insure the necessary intimate contact between the plates and separators. Also, the oxygen initially produced at the positive plates must be contained in the cells under pressure (typically 0.5 to 8.0 psig) so that it contacts the negative plates to effect the oxygen cycle.
Unfortunately, the elevated internal pressures, necessary for the oxygen cycle, cause conventional containers to bulge, thus, causing a relaxation of the compressive force in the end cells. Consequently, the intimate surface contact between the separators and positive and negative plates in the end cells is reduced causing battery efficiency to be significantly reduced.
Sealed lead-acid batteries presently being produced attempt to nullify bulging through the use of stiffer materials less prone to bulge, stiffening ribs incorporated in the container end wall design, or thicker container end wall construction. While these approaches may offer some improvement, none of these techniques are entirely effective. Not only do such configurations increase the cost of manufacturing the batteries due to the higher cost of stiffer materials or using more of conventional materials, but they also increase the weight of the batteries, another important consideration in battery design.